I tried to update my North American/Canada Maps on Nuvi 760(lifetime Map program)but it keeps advising that a SD 3 GB or larger Card is required. Mine is a 4GB. I removed the SD Card and re-inserted but it repeats the 3 GB message. Why will it not proceed with the update? The SD Card is 1 GB over the minimum requested.

You can try backing the SD card contents onto your PC, then reformatting the card before putting it back into the GPS. This may reset how the system sees the card.
DO NOT FORGET TO BACK UP CONTENTS FIRST!!